Automatic specimen polishing machine and method

ABSTRACT

An abrading apparatus for polishing specimens has a non-rotating polishing head, a first high speed motion to effect local orbital polishing or abrading motion and one second relatively slow speed translational motion to effect surface coverage of the specimen to be polished or abraded over the surface of the polishing head. The loading of the specimen may be continuously changed during operation of the machine.

United States Patent [191 Roth [ 1 Sept. 23, 1975 AUTOMATIC SPECIMEN POLISHING MACHINE AND METHOD [75] inventor: Kurt H. Roth, Mississauga, Canada [73] Assignee: Buehler Ltd., Evanston, Ill.

[22] Filed: Dec. 26, 1973 [21] Appl. No: 427,749

Related U.S. Application Data [63] Continuation-impart of Ser. No. 289,108, Sept. 14,

1972, abandoned.

[52] U.S. Cl. 51/154; 51/120; 51/161; 51/318 [51] Int. Cl. B24B 37/04; B24B 1/00 [58] Fieid of Search 51/119, 120, 149, 150,

51/154, 161, 162, 237 M, 54,13l,133,124 L, 28,317,318, 283

[56] References Cited UNITED STATES PATENTS 971,861 10/1910 Nickerson 51/133 1,541,001 6/1925 Scarles 51/161 1,577,137 3/1926 Maynard 51/133 3,691,697 9/1972 Bender 51/1659 Primary Examiner-Al Lawrence Smith Assistant Examiner-Nicholas P. Godici Attorney, Agent, or Firm-Charles F. Pigott, Jr.

[57] ABSTRACT An abrading apparatus for polishing specimens has a non-rotating polishing head, a first high speed motion to effect local orbital polishing or abrading motion and one second relatively slow speed translational motion to effect surface coverage of the specimen to be polished or abraded over the surface of the polishing head. The loading of the specimen may be continuously changed during operation of the machine.

10 Claims, 9 Drawing Figures US Patent Sept. 23,1975 Sheet 1 of3 3,906,678

US Patent Sept. 23,1975 Sheet 2 of3 3,906,678

US Patent Sept. 23,1975 Sheet 3 of3 3,906,678

AUTOMATIC SPECIMEN POLISHING MACHINE AND METHOD RELATED CASE This is a continuation-in-part application of my copending application, Ser. No. 289,108, filedSept. 14, 1972, entitled Automatic Polishing Machines, now abandoned.

BACKGROUND OF THE INVENTION This invention is directed to an abrading system, in particular a method of effecting controlled abrasion such as polishing of a specimen over a predetermined workpiece area, and apparatus for carrying out the method.

In effecting controlled abrasion of a surface of a workpiece for progressive polishing thereof, the most well known methods involve the use of a rotatable work table or head, to which is attached a suitable abrading surface, polishing cloth, or to which an abrading, grinding or polishing media is applied. One of the problems inherent in such prior art abrading process is the dispersion of fluent abrading media from the working zone, primarily under the influence of centrifugal forces. Owing to-dynamic limitations it is not generally possible to utilize satisfactorily thick pastes in cooperation with rotating wheels to produce fine polishing.

The utilization in many prior processes of circular generating actions tends to produce a marked directional characteristic of abrasion that generally prove undesirable and difficult to remove as successively reduced abrasion proceeds.

SUMMARY OF THE INVENTION The present invention provides a non-rotatory translational primary action at relatively high speed that secures effective abrasion and a secondary displacement or translational action to produce travel of the specimen in controlled relation over the surface of the polishing head so that the surface of the workpiece is subject to a composite motion having modifying components of direction and speed to produce abrasion along a continuously changing direction of motion, thereby minimizing directional grooving effects.

Thus there is'provided a method of abrading the surface of a specimen by providing controlled polishing motion between two surfaces of markedly different size mounted in controlled abrading relation, including the steps of applying a predetermined continuously variable load to urge the surface together in abrading relation, imparting a high speed relative motion of short path in repetitive abrading relation to one body and simultaneously producing relative movement at a slower rate between the bodies whereby the abrading body has a rapid working stroke continuously varying in direction at a slow rate.

A further characteristic of the present invention is the provision of a stationary cup-like abrasion head to contain a sensible pocket of abrading media or lubricant, thereby assuring substantially constant conditions of media or lubricant feed.

The controlled action of a small working head with a relatively fast working action and controlled slower translational motion permits polishing of irregular shapes in a satisfactory manner.

Owing to the regulated distribution of the working strokes over the surface of a workpiece the present method and apparatus may also be used satisfactorily with less hard materials, without producing abrasion in an irregular or eroding style.

The present invention thus provides an abrading mechanism having first and second surface contacting means; loading means to urge the two surface means toward each other in controlled abrading relation; drive means to drive one surface means in abrading relation with the other surface means including a first high speed motion inducing mechanism to produce localized high speed abrading motion between the two surface means; and one second 'motion inducing mechanism to extend the area of action of the first mechanism whereby a composite high speed abrading motion and lower speed translation motion are imparted between the two surface means. Y.

Another characteristic of the present invention is the application of constantly changing controlled loading of the working head, permitting in the extreme the displacement of the specimen from the polishing surface, under the influence of uninterrupted hydrodynamic lift, through the presence and action of fluid abrasive media within the cup-like working head. Thus in instances where the working loading produces this condition, polishing motion proceeds underthe influence of viscosity of the polishing paste medium, while the displacement of the specimen from the polishing surface prevents entrapment of abrading particles between the head and the worked surface, so that gouging of scratches is substantially precluded.

In addition to the foregoing characteristics, thepresent invention provides certain benefits in controlling the application of the working head to the workpiece by; retaining polishing paste within the working jhe ad recessed face; and maintaining in well mixed condition the material within the face recess, including providing dispersion of abrasion debris in the polish media.

The provision of continuously variable working load enables a desired sequence to be followedthe minimum maintenance, so that a complete sequence from abrasive cutting to fluid friction polishing under solely hydro dynamic forces can be effected without the necessity of changing grades of abrasive or changing from abrasive polish, as in the prior art.

The present invention permits effective abrading and more particularly polishing at linear surface speeds in the order of feet per minute, as contrasted with ,orthodox lapping wheel motion such as where a 200 rpm low speed drive provides lapping speeds of 400 feetper minute.

A further and unobvious advantage of the present invention is that the imposition of a secondary displacement motion on a primary abrading motion changes the selective direction of advance of the working head, so that there is produced continuous changes in the cutting points with consequent enhancement of the cutting action over the uniform and substantially unidirectional motion of prior wheel arrangements.

The use of a small working head and movingonly the specimen provides low inertia loads and small surfaces, with consequently reduced structure and material costs.

BRIEF DESCRIPTION OF THE DRAWINGS Certain embodiments of the present invention are described, reference being made to the accompanying drawings, wherein:

FIG. 1 is a general view of an embodiment of the present invention, shown in part section;

FIG. 2 is a view taken at 2 of FIG. 1, showing the working head and a related workpiece;

FIG. 3 is a plan view of the locus of a typical working path;

FIG. 4 is a section view at 4-4 of FIG. 2;

FIG. 5 is a view of the apparatus from one side, with the near casing wall removed for clarity;

FIG. 6 is a front section taken at 6-6 of FIG. 5;

FIG. 7 is a front section taken at 7--7 of FIG. 5; and

FIG. 8 is a view taken from location 8 of FIG. 5, showing a general view of the working head load adjusting mechanism.

FIG. 9 is a top perspective view of an abrasing chamber showing three workpiece holders therein attached to the driving head for coincident abrading action.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1, 2, 3 and 4 the apparatus comprises generally a cabinet 10 having a cover plate 10' which is therein shown as providing a flat-topped base for a stationary abrading chamber 12 over which is stretched a cloth 13 or other suitable material held taut by means of a ring 14 lying above the sheet 13 and held in clamping position by means of screws 14' which extend through the cloth and screw into base 12.

The member 12 constitutes a chamber for an abrasive substance 15 which may be of any suitable consistency or nature. It is placed in position on top 10' so that boss 23 (FIG. 5) engages with a corresponding rcccss 21 in member 12. Member 12 is prevented from rotating by friction between its base and top 10'.

In FIG. 4 there is shown a disc, or block, having a specimen embedded in its lower side as is conventional practice, exposing the lower surface to the polishing action.

An arm 18 driven by suitable motors 26 and has a head 16 extending loosely into an opening in the top surface of the specimen mount 20 so that in operation the specimen mount 20 will describe a course corresponding substantially equivalent to that shown in FIG. 3 which represents a typical composite orbital locus of a working head center.

In FIG. 1 can be seen an electric motor 26 to drive one of the orbital drive means by way of a belt 28.

Referring to FIGS. 5, 6 and 7, motors 26 and 30 drive belts 28 and 32 respectively. An actuating axle 34 is mounted eccentrically in relation to an inner pulley 36 that is driven by the belt 28. The inner pulley 36 is in turn mounted eccentrically in relation to the outer pulley 38 that is driven by the belt 32.

The output pulley 40 of the motor 26 is driven directly by the motor, whereas in the case of the motor 30 an intermediate reduction gear 42 provides a low speed output to the respective output pulley 50.

An outer bearing 60 of the transmission is mounted on studs 62 by way of clips 64 and nuts 66, or by some other convenient means in relation to the top surface.

The axle portion 34 of the arm 18 is biased downwardly by tension spring 68., the lower end of which is carried by a lever 70. The lever 70 pivots about a post 72 and extends through the front of the cabinet to a handle 74 (FIG. 1 and 5). The lever 70 engages a threaded shaft 76 that is selectively rotated by a servo motor 80 having a control 82 therefore annotated au- Ill tomatic" in FIG. 1 to indicate that automatic decrease in abrading load is being applied through the spring 68 by continual raising of the arm through rotation of the shaft 76. A spring 77 secures a knife edge portion 79 of lever 70 in engaged relation with the threads of shaft 76, to permit hand adjustment by way of the handle 74.

The frame 17 having a top member or arm 18 and a bottom member 19 is supported laterally by guides 90 for sliding and pivotal oscillating motion relative thereto. Thus the composite motion applied by the two pulleys 36, 38 to the axle 34 is transferred as oscillating and sliding motion to the frame 17, which substantially duplicates the motion of the axle 34 at the head 16.

Similarly the tension provided by the spring 68 acting on axle 34 appears as downward loading on the head 16.

The hydrodynamic forces serving to raise the head 16, during operation of the apparatus. offset at least in part the load provided by the spring 68.

The eccentricity of the mounting of the pulley 36 may occasion the need of elastic belt 28 or the adoption of a resiliently loaded idler (not shown) to maintain the engagement of the belt drive.

FIG. 9 indicates a method whereby the utility of the device may be extended to polishing more than one workpiece or specimen simultaneously by using a head 16' with multiple extensions and cup dimension to suit.

The method and apparatus of the present invention are especially adapted for polishing specimens for microstructural analysis such as in the fields of metallurgy, pctrography and mineralogy. Such specimens may comprise soft materials such as lead alloys, brittle materials such as coal and oxide coatings, sensitive materials such as austenitic stainless steel and zinc alloys, and hard materials such as carbides and iron ore pellets. In accordance with the present invention a specimen can be polished for microstruetural analysis in a relatively short time, generally in less than ten minutes. The lapping cup 12 is maintained in a stationary position by any suitable means, and the specimen mounting member 20 is moved over the flat polishing surface comprising the base of the lapping cup 12 over which is fastened the polishing cloth 15.

As a result of the motion imparted to the specimen as described hereinabove, the abrasive slurry in the lapping cup is continuously drawn toward the center of the polishing surface, thus assuring that an adequate amount of slurry is present at the abrasion interface between the spccimen and the polishing surface and not permitted to accumulate at the outer periphery of the polishing surface. The direction of movement of the specimen changes rapidly subjecting each and every point on the specimen surface to essentially identical, multidirectional abrasion. By way of example, in accordance with one preferred embodiment, the direction of abrasion changes at a rate of 360 in one-tenth of a second, while the speed of abrasion changes cyclically between 46 and feet per minute.

In the polishing of specimens for microstructural analysis, the lapping cup 12 is particularly useful and practical, since it is readily cleaned and convenient for storage and use. Because such cups are small and inexpensive, it is practical to reserve a separate cup for each specific polishing requirement, i.e., nap surface. 0.3 micron alumina, steel. etc. A cup, once set up. is simply taken from its storage container, fresh abrasive is added, and the cup may then be uscd immediately with a new specimenpOn completion of a polishing operation, the cup may simply be rinsed and returned to storage. While it is possible to use a charge of abrasive slurry several times, it is generally preferable to rinse the cup clean after each use and supply fresh abrasive for the next polishing operation. A

In accordance with one preferred embodiment, the flat polishing area is approximately 2% inch in diameter. Due to the use of a small polishing area, and the movement of the specimen over the major portion of the polishing surface, intense mixing action between the specimen and the side walls of the lapping cup prevents settling of the abrasive slurry and at the same time disperses polishing debris uniformly into the total abrasive mass. As a result, the abrasion characteristics of the slurry remain quite uniform throughout a polishing cycle, and of course the slurry is fully contained so that no loss occurs.

In polishing of specimens for microstructural analysis, the specimen load can be set at any value up to about eight pounds, and can be made to reduce continuously at a predetermined rate by the automatic mech anism described hereinabove. By the use of such a procedure, it is possible to achieve ultrafine finishes considerably better than can be achieved using conventional contact polishing. By using a viscous paste in the lapping cup and applying a low load, the hydrodynamic lift induced by the continuous polishing motion is capable of prolonged automatic skid polishing by floating the specimen off the polishing cloth. Moreover, polishing under a slowly reducing load offers several advantages. A polishing cycle generally requires high initial abrasion rates to remove strained material remaining from a previous coarser step. However, once such material is removed, it becomes more important to minimize the introduction of new polishing damage. If the rate of material removal is sufficiently fast to remove strained layers, while new damage is done at a decreas ing rate and to a decreasing depth due to lower pressures and less severe polishing, the final damaged layer will be minimized. The above-mentioned skid polishing will occur automatically at the end of a reducing pressure cycle if the abrasive slurry or paste is sufficiently viscous, and if the downwardly load applied to the specimen is reduced to an amount less than the upward force on the specimen induced by the aforementioned hydrodynamic lift effect. It is therefore possible in accordance with the present invention to provide a fully automatic cycle beginning with high abrasion rates and ending in skid polishing.

A specimen 24 is embedded in the face of the specimen mounting disc 20. The surface of the specimen is then prepared by conventional pre-grinding. Thereafter, an appropriate lapping cup 12 is selected from storage, and abrasive and fluid are added to prepare a suitable slurry or pastev The specimen is then placed in the cup 12 and the cup is placed on the machine and mounted so as to be stationary. A selected initial load is applied to the specimen, and the apparatus is then operated in the manner described hereinabove. It has been found that a wide range of materials can be polished for microstructural analysis by using only two polishing stages subsequent to conventional pre grinding to 600 grit wet emery. For example, for a first stage I have used 0.3 micron alumina in distilled water in a polishing cycle lasting 1 to 3 minutes, and in a sec- 0nd stage I have used colloidal magnesia in distilled water in a polishing cycle lasting 2 to 4 minutes. In many cases, one stage alone will produce a satisfactorily polished specimen surface. The foregoing second stage has been used to remove fine scratches, especially from a specimen made of a soft material.

It is important to understand that in polishing the surface of a specimen for purposes of microstructural analysis, it is of critical importance that, after polishing, the microstructure of the specimen surface will be the same as prior to polishing. Heretofore it has not been practical with automatic polishing equipment to effect abrading or polishing of a specimen while preserving the integrity of the microstructure at the surface of the specimen, and as a result the overwhelming preponderance of all such polishing has been effected by hand, i.e., by manually holding a specimen in engagement with a rotating lap.

In accordance with the present invention, the polishing surface is not rotated as a conventional lap, but rather is mounted in a stationary manner. By thus eliminating the usual centrifugal force, there is no tendency for the abrasive slurry to collect out at the periphery of the polishing surface. A rapid localized circular motion is applied to the specimen, and simultaneously therewith the specimen is caused to undergo a translatory circular motion or progression around an area on the polishing surface substantially larger than the areaof the specimen to be polished. One specific'example of such composite motion is illustrated in FIG. 3 of the Drawings.

The foregoing motion utilized in conjunction with the cup-shaped member 12 having a stationary flat polishing surface, with abrasive slurry contained thereon, affords numerous advantages in the polishing of specimens for microstructural analysis. The rapidly changing direction of the specimen as it moves over the polishing surface renders the abrasive action more efficient and enables the use of lower speeds while still cffecting a polishing operation in less time than with a conventional lap or the like, and without altering the microstructure of the surface of the specimen.

A polishing apparatus for the polishing of specimens for microstructural analysis will normally be used with specimens of a wide variety of different materials. Most such materials differ from one another with respect to optimum cutting rates or abrasion speeds, and in many instances the optimum speed for a given material will be unkown. The apparatus and method of the present invention combine a localized circular motion with a translational circular progression resulting in a motion which is the composite of the two individual motions, and such individual motions will be repeatedly added and then subtracted from one another depending upon whether the directional characteristics of the respective motions are additive or whether they offset one another at any given instant. The result is that the resultant speed of the composite motion imparted to the specimen will vary rapidly between a maximum and a minimum speed so as to automatically provide a range of speeds. It has been found that such an automatic variation of the speed of abrasion produces exceptionally superior polishing results with a wide variety of different specimens having significantly different polishing characteristics.

It has been found that in the practice of the present invention the specimen will tend to rotate about its own axis, i.e., about the axis of the vertical portion 16 of the arm 18 which engages with the specimen mounting member, as it is moved over the polishing surface in the manner described herein. Such additional rotation of the specimen about its own axis is not believed essential to the utility of the invention, but is believed to increase the efficiency of the polishing or abrading action.

It has been established that a self-induced hydrodynamic lift effect can be achieved with the automatic polishing apparatus of the present invention. When such an effect occurs, referred to hereinabove as skid polishing, the specimen in effect floats on the abrasive slurry without contacting the polishing surface. Under such a condition, the polishing action will continue to occur due to the viscosity of the abrasive slurry, and yet there is eliminated any possibility of gouging scratches in the specimen by abrasive particles because the latter are free to pass under the specimen and over the polishing surface without becoming entrapped therebetween. Such a hydrodynamic lift effect is achieved in accordance with the present invention as a result of the continuous composite motion applied to the specimen, in combination with the continuous reduction of the vertical load on the specimen, in combination with a downward load on the specimen which is somewhat less than the upward force induced due to the effect of hydrodynamic lift.

A more substantial load is desirable in the beginning of the polishing operation to effect a high rate of abrasion. By subsequently reducing the load, it has been found possible to achieve the foregoing hydrodynamic lift effect to minimize damage to the specimen during the latter portion of the polishing cycle. However, it has been found that a single reduction in the load is not sufficient to achieve the full advantages of the invention. Thus, only by a substantially continuous reduction of the specimen load during a polishing operation is it possible to achieve conditions where the rate of material removal due to abrasion exceeds the damage necessarily inflicted on the specimen during such removal. By such substantially continuous reduction of the specimen load, it is possible to achieve a polishing operation where the amount of damage to the specimen is continuously reducing, whereby at the conclusion of a polishing operation, the amount of damage to the polished specimen will be at a minimum.

I claim:

1. An automatic abrading apparatus for polishing a specimen, said apparatus comprising, in combination, a generally cup-shaped member having an upper recess for containing an abrasive slurry or the like, said cupshaped member being mounted so as to be substantially stationary, and said recess having a generally horizontal base comprising a substantially flat polishing surface having an area substantially greater than the surface area of a specimen to be polished, a drive arm extending over said recess and having an end portion engageable with a specimen disposed in said recess on said polishing surface. said drive arm being mounted for movement in any direction in a horizontal plane for moving a specimen over said flat polishing surface in rapidly changing directions and at rapidly changing speeds, drive means connected with said drive arm for actuating the latter, said drive means including a first drive member imparting a relatively high speed circular motion to said drive arm end portion, and a second drive member simultaneously imparting a relatively low speed motion to said drive arm end portion. said low speed motion being a circular progression around a major portion of the area of said polishing surface whereby the movement of said drive arm end portion is the composite of the motions effected by said first and second drive means, and loading means for producing a downward load on said drive arm end portion thereby imparting a desired vertical load on a specimen engaged therewith.

2. An automatic abrading apparatus as defined in claim 1, wherein said drive arm is disposed generally horizontally, and bearing means supporting said drive arm intermediate the ends thereof, said drive arm being longitudinally slidable relative to said bearing means and being pivotable about a vertical axis at said bearing means.

3. An automatic abrading apparatus as defined in claim 1 wherein said first drive member comprises a first rotary eccentric drive connected with said drive arm, and said second drive member comprises a second rotary eccentric drive member which imparts a circular progression to said first rotary eccentric drive, said first drive member being carried by said second drive member for movement therewith, and said first drive member also being movable relative to said second drive member.

4. An automatic abrading apparatus as defined in claim 1 where said loading means comprises a resilient load-applying member connected with said drive arm, and load control means operating on said load-applying member for substantially continuously reducing the vertical loading on a specimen during a polishing operation.

5. A method of polishing the surface of a specimen comprising the steps of supporting a specimen to be polished on the top ofa stationary fiat polishing surface having an area substantially exceeding the surface area of the specimen and having abrasive slurry or the like thereon, engaging the specimen from above and applying two simultaneous motions to said specimen in a horizontal plane, one motion being a relatively high speed localized circular motion and the second motion being a relatively slow circular progression or translatory motion, said second motion causing the specimen to traverse over the major portion of the area of said polishing surface, said two circular motions being superimposed upon one another to impart a composite motion to said specimen causing the latter to travel along a looping path which repeatedly crosses itself as the specimen progresses around said polishing surface, and containing substantially all of said abrasive slurry on said polishing surface throughout the polishing operation.

6. A method of polishing the surface of a specimen as defined in claim 5 including the steps of applying a downward load to said specimen during a polishing operation and substantially continuously reducing said downward load during said polishing operation while the specimen progresses repeatedly around said polishing surface.

7. A method of polishing the surface of a specimen as defined in claim 5 including the steps of applying a downward load to said specimen during a polishing operation and reducing the load to a magnitude sufficiently low to permit the specimen to lift off the polishing surface due to hydrodynamic lift induced through the viscosity of the finite quantity of abrasive slurry contained on said polishing surface and said composite motion of the specimen. and continuing the polishing operation under such hydrodynamic lift conditions thereby effecting polishing of said specimen due to abrasive friction without physical contact between the specimen and the polishing surface while the specimen progresses repeatedly around said polishing surface.

8. A method of polishing a specimen for microstructural analysis comprising the steps of pro-grinding a substantially flat surface on the specimen, supporting the specimen with said flat specimen surface disposed on a stationary flat polishing surface having an area substantially exceeding the area of said specimen surface, supplying abrasive slurry to said polishing surface, engaging the specimen from above and applying two simultaneous motions to said specimen in a horizontal plane, one motion being a relatively high speed localized circular motion and the second motion being a relatively slow circular progression or translatory motion, said second motion causing the specimen to traverse over the major portion of the area of said polishing surface, said two circular motions being superimposed upon one another to impart a composite motion to said specimen causing the latter to travel along a looping path which repeatedly crosses itself as the specimen progresses around said polishing surface, applying a downward load to said specimen during the polishing operation, and containing substantially all of said abrasive slurry on said polishing surface throughout said polishing operation.

9. A method of polishing a specimen for microstructural analysis as defined in claim 8 including the step of substantially continuously reducing said downward load on the specimen during said polishing operation whereby abrasion intensity decreases continuously throughout said polishing operation.

10. A method of polishing a specimen for microstructural analysis as defined in claim 8 including the step of reducing said downward load on said specimen during said polishing operation until said load is less than the upward vertical force induced on said specimen due to hydrodynamic lift, whereby thereafter said specimen will in effect float on said abrasive slurry during subsequent polishing of said specimen. 

1. An automatic abrading apparatus for polishing a specimen, said apparatus comprising, in combination, a generally cup-shaped member having an upper recess for containing aN abrasive slurry or the like, said cup-shaped member being mounted so as to be substantially stationary, and said recess having a generally horizontal base comprising a substantially flat polishing surface having an area substantially greater than the surface area of a specimen to be polished, a drive arm extending over said recess and having an end portion engageable with a specimen disposed in said recess on said polishing surface, said drive arm being mounted for movement in any direction in a horizontal plane for moving a specimen over said flat polishing surface in rapidly changing directions and at rapidly changing speeds, drive means connected with said drive arm for actuating the latter, said drive means including a first drive member imparting a relatively high speed circular motion to said drive arm end portion, and a second drive member simultaneously imparting a relatively low speed motion to said drive arm end portion, said low speed motion being a circular progression around a major portion of the area of said polishing surface whereby the movement of said drive arm end portion is the composite of the motions effected by said first and second drive means, and loading means for producing a downward load on said drive arm end portion thereby imparting a desired vertical load on a specimen engaged therewith.
 2. An automatic abrading apparatus as defined in claim 1, wherein said drive arm is disposed generally horizontally, and bearing means supporting said drive arm intermediate the ends thereof, said drive arm being longitudinally slidable relative to said bearing means and being pivotable about a vertical axis at said bearing means.
 3. An automatic abrading apparatus as defined in claim 1 wherein said first drive member comprises a first rotary eccentric drive connected with said drive arm, and said second drive member comprises a second rotary eccentric drive member which imparts a circular progression to said first rotary eccentric drive, said first drive member being carried by said second drive member for movement therewith, and said first drive member also being movable relative to said second drive member.
 4. An automatic abrading apparatus as defined in claim 1 where said loading means comprises a resilient load-applying member connected with said drive arm, and load control means operating on said load-applying member for substantially continuously reducing the vertical loading on a specimen during a polishing operation.
 5. A method of polishing the surface of a specimen comprising the steps of supporting a specimen to be polished on the top of a stationary flat polishing surface having an area substantially exceeding the surface area of the specimen and having abrasive slurry or the like thereon, engaging the specimen from above and applying two simultaneous motions to said specimen in a horizontal plane, one motion being a relatively high speed localized circular motion and the second motion being a relatively slow circular progression or translatory motion, said second motion causing the specimen to traverse over the major portion of the area of said polishing surface, said two circular motions being superimposed upon one another to impart a composite motion to said specimen causing the latter to travel along a looping path which repeatedly crosses itself as the specimen progresses around said polishing surface, and containing substantially all of said abrasive slurry on said polishing surface throughout the polishing operation.
 6. A method of polishing the surface of a specimen as defined in claim 5 including the steps of applying a downward load to said specimen during a polishing operation and substantially continuously reducing said downward load during said polishing operation while the specimen progresses repeatedly around said polishing surface.
 7. A method of polishing the surface of a specimen as defined in claim 5 including the steps of applying a downward load to said specimen during a polishing operation and reducing the load to a magnitude sufficiently low to permit the specimen to lift off the polishing surface due to hydrodynamic lift induced through the viscosity of the finite quantity of abrasive slurry contained on said polishing surface and said composite motion of the specimen, and continuing the polishing operation under such hydrodynamic lift conditions thereby effecting polishing of said specimen due to abrasive friction without physical contact between the specimen and the polishing surface while the specimen progresses repeatedly around said polishing surface.
 8. A method of polishing a specimen for microstructural analysis comprising the steps of pre-grinding a substantially flat surface on the specimen, supporting the specimen with said flat specimen surface disposed on a stationary flat polishing surface having an area substantially exceeding the area of said specimen surface, supplying abrasive slurry to said polishing surface, engaging the specimen from above and applying two simultaneous motions to said specimen in a horizontal plane, one motion being a relatively high speed localized circular motion and the second motion being a relatively slow circular progression or translatory motion, said second motion causing the specimen to traverse over the major portion of the area of said polishing surface, said two circular motions being superimposed upon one another to impart a composite motion to said specimen causing the latter to travel along a looping path which repeatedly crosses itself as the specimen progresses around said polishing surface, applying a downward load to said specimen during the polishing operation, and containing substantially all of said abrasive slurry on said polishing surface throughout said polishing operation.
 9. A method of polishing a specimen for microstructural analysis as defined in claim 8 including the step of substantially continuously reducing said downward load on the specimen during said polishing operation whereby abrasion intensity decreases continuously throughout said polishing operation.
 10. A method of polishing a specimen for microstructural analysis as defined in claim 8 including the step of reducing said downward load on said specimen during said polishing operation until said load is less than the upward vertical force induced on said specimen due to hydrodynamic lift, whereby thereafter said specimen will in effect float on said abrasive slurry during subsequent polishing of said specimen. 